1. Technical Field
The present disclosure relates to pyridinium-based cages for the capture of polycyclic aromatic hydrocarbons.
2. Description of Related Art
Polycyclic aromatic hydrocarbons (PAHs or PAH compounds)—molecules which consist of two or more fused aromatic rings—are commonly found in natural crude oil deposits and also arise from anthropogenic processes during the incomplete combustion of carbon-based materials. The carcinogenic properties of PAHs have long been known and the pathways by which they cause mutagenesis are well documented. Not only are they prevalent in the environment, but they also persist on account of their low solubilities in water. The smaller PAHs, however, such as naphthalene, have a slightly higher water solubility and so are apt to leach out into the waterways. Yet, despite this situation, and its implications in relation to several disease states, naphthalene is produced annually on a massive scale. Although numerous hosts, with affinities for PAHs, based on dispersion forces and solvophobic effects, have been reported, the donor-acceptor interactions that have come into play with π-electron-deficient hosts, lead to higher binding affinities for PAHs, even in organic solvents. To date, few molecular compounds exist for eliminating PAHs from the environment.
N,N′-diazamacrobicyclic polyethers (cryptands) are three-dimensional analogues of the crown ethers. The N,N′-diazamacrobicyclic polyethers bind Group 1A and IIA metal cations so strongly that their 1:1 complexes became known as cryptates1. While the progression from crown ethers to cryptands occurred rapidly, it took quite a few years for the more highly designed spherands2, carcerands3 and hemicarcerands4,5 to make their entry on to the scene as hosts with concave inner surfaces that provide convergent recognition sites for the complexation of guests in the form of ions and neutral molecules with divergent binding sites. These early developments in host-guest chemistry laid the foundations for the design and synthesis of cage-like host molecules with constitutions ranging from being wholly organic to metal-coordinated. These unnatural products, that fall under the umbrella of molecular cages, have been designed and synthesized for a vast range of different reasons including (i) exploring and exploiting their geometries, (ii) studying their properties as molecular magnets, (iii) employing them as molecular vehicles in the biomedical arena, and (iv) using them to modulate and catalyze chemical reactions.
Applicants reported6 on the efficient template-directed synthesis' of higher homologues of cyclobis(paraquat-p-phenylene) (CBPQT4+; FIG. 1: structure (I)), resulting from extending both its bipyridinium units by inserting a p-phenylene ring in a stepwise fashion between the two pyridinium rings to produce extended tetracationic cyclophanes of structure (II), denoted herein as ExnBox4+ where n=0-3 (FIG. 1: structure (II)). The tetracationic cyclophanes of structure (II) having n=1 is referenced herein as ExBox4+ (FIG. 1: structure (IIA). These structures are referred to as “two-dimensional boxes” that bind poorly to PAHs; thus, there is a need for new compounds having greater binding affinity for PAHs.